Object identification by emission polarization

ABSTRACT

Metallic objects are identifiable by the degree of polarization of infrared radiation emitted therefrom. To identify a metallic object from a nonmetallic object by radiation emitted therefrom, three linearly polarized detectors are positioned to be responsive to the emitted radiation. Each of these three detectors is responsive to radiation from the object along a different plane of polarization. Output signals from the three detectors are processed in a system to produce a signal equal to the square of the degree of polarization. Smooth objects are distinguishable from rough objects in the same field of view by determining the degree of polarization of a light beam directed to and reflected from the objects. By knowing the polarization axis of the light waves at the source, the output signals of two detectors may be combined to highlight in a display smooth objects in a field of view that contains both smooth and rough objects.

United States Patent Covault July 24, 1973 OBJECT IDENTIFICATION BYEMISSION Primary Examiner-James W. Lawrence POLARIZATION AssistantExaminer-D. Nelms H I Attbrney-Samuel M. Mims, .lr., Ren E. Grossman[75] Inventor: Dennis O. C0vault,Garland,Tex. t a1,

[73] Assignee: Texas lnstruments lncorporated, 57 ABSTRACT Dallas, Tex.

Metallic objects are identifiable by the degree of polarl Flled! J 1971ization of infrared radiation emitted therefrom. To [211 App. NOJ150,739 identify a metallic object from a nonmetallic object byradiation emitted therefrom, three linearly polarized Related US.Application Data detectors are positioned to be responsive to theemitted [62] Division of Ser. No. 889,406, D 31, 1969, P N radiation.Each of these three detectors is responsive to 3,631,254. radiation fromthe object along a different plane of polarization. Output signals fromthe three detectors are [52] US. Cl 250/225, 356/116, 356/118 processedin a system to produce a signal equal to the [51] Int. Cl. G02t l/18square of the degree of polarization. Smooth objects [58] Field ofSearch 250/225; 350/147; are distinguishable from rough objects in thesame field 356/114, 115, 116, 117, 118, 1 19 of view by determining thedegree of polarization of a light beam directed to and reflected fromthe objects. [56] References Cited By knowing the polarization axis ofthe light waves at UNITED STATES PATENTS the source, the output signalsof two detectors may be combined to highlight in a display smoothobjects in a 313321312 31132? 3232231111133311131: "1:: 32353? i fold ofvow toot oooooioo oooh oooooh ooo rough 3,060,793 10/1962 Wells 356/118j 3,565,248 Messerschmidt Claims, 8 Drawing Figures VIDEO PROCESSOR CRTDISPLAY PAIENIEDJUI 24 I975 POLARIZATION SHEET 3 0F 3 EMISSIONPOLARIZATION AT IO.|7 MICRONS (0.35 MICRON BANDWIDTH) FOR: A POLISHEDALUMINUM AT 7lC lo C1 TEFLON AT 53C PROCESSOR VIDEO I46 T f 150 Z 0 cmTHRESHOLD GATE 4 OBJECT IDENTIFICATION BY EMISSION POLARIZATION Thisapplication is a division of application Ser. No. 889,406, filed Dec.31, 1969, now US. Pat. No. 3,631,254.

This invention relates to object identification, and more particularlyto object identification by means of the degree of polarization ofenergy received from an object.

Radiation continuously emitted from all objects results from theacceleration of electrical charges within the material. It was earlyrecognized that this emission radiation was polarized to a high degreefor metals and that nonmetals had a low degree of emission polarization.Thermal radiation emanating from within an object is unpolarized untilit crosses the surface interface. The components of the radiation in theparallel and perpendicular emission planes are transmitted differentlyin crossing the interface. This difference is a function of theelectrical constants of the material and the direction of emission withrespect to the normal to the surface, as predicted by Fresnel sequations. It has been exhibited both experimentally and theoreticallythat the emission polarization is due to refraction of the radiation atthe surface of the material. Infrared emission for metallic objects hasbeen shown to be highly polarized, whereas that from non-metallicobjects show a low degree of polarization.

In addition to emitted radiation polarization from an object, it hasalso been shown that the surface roughness of an object has a directeffect on the changes in polarization of reflected energy. Polarizationscattering matrix theory, which is valid at optical wavelengths althoughit has been most frequently applied at microwave and radio wavelengths,shows that a rough surfaced object depolarizes an incident beam, whereasa smooth surface can polarize an unpolarized incident beem or change thepolarization of a polarized impinging light beam. This characteristic ofan object, its signature, is wavelength dependent. That is, as thewavelength of the incident radiation increases, a given surface willappear to become smoother. If the roughness of the surface can beindicated by a length parameter, then the wavelength dependence will bemost pronounced for surfaces where this parameter is between the maximumand minimum wavelength limits of the illuminating source.

A feature of this invention is to identify an object by the degree ofpolarization of radiation emitting therefrom. Another feature of thisinvention is to provide for identification of an object by measuring thedegree of polarization of light reflected therefrom. A further featureof this invention is to provide for identification of an object asmetallic or nonmetallic by the degree of polarization of radiationemitting therefrom. Still another feature of this invention is toprovide for object identification as to a smooth surface or a roughsurface by the degree of polarization of light reflected therefrom. Astill further feature of this invention is to provide for objectidentification by determining the degree of polarization or radiationemitting therefrom by measurements along at least three different planesof polarization. An additional feature of this invention is for objectidentification by measuring the polarization scattering of a polarizedlight beam reflected from an object to be identified.

In accordance with one embodiment of the invention, an object isidentified by measuring the polarized radiometric intensity along atleast three different planes of polarization with respect to the object.Radiometric intensity measurements from each of the at least threedifferent planes of polarization are summed to obtain a total intensitymeasurement. This total intensity measurement is further processed bygenerating a signal representing the square thereof. Additionalprocessing of the radiometric intensity measurements resulting from eachof the planes of polarization produces a signal equal to the square ofthe degree of polarization of radiation from the target times the squareof the total radiation intensity. This last signal is combined with thesquare of the total intensity measurement in a manner that results in agenerated signal equal to the square of the degree of polarization ofradiation from an object.

In accordance with another embodiment of the invention, an object isidentified as smooth or rough, relatively speaking, by illuminationthereof from a radiant energy source polarized along a known axis.Measurements are made of the scattering of the polarized lightilluminating the object from along at least two polarization planes bylinearly polarized sensitive detectors at angles relative to thepolarization axis of the illuminating radiation. These measurements aresummed to pr0- duce a total intensity measurement of the polarized lightreflected from the object. In addition, a signal is generated which isequal to the total intensity times the degree of polarization of thereflected radiation in excess of a threshold. The threshold isestablished to distinguish between smooth and rough objects. Additionalprocessing of the total intensity measurement signal and the generatedsignal in excess of the threshold, produces a display signal that may beused to identify an object as smooth or rough by highlighting the smoothobjects in a display device.

A more complete understanding of the invention and its advantages willbe apparent from the specification and claims and from the accompanyingdrawings illustrative of the invention.

Referring to the drawings:

FIG. 1 is a block diagram of a system for measuring the degree ofpolarization of an object from the radiation emitting therefrom;

FIG. 2 is a polarization ellipse that represents the path that anelectric vector of radiation sweeps, propagating normal to the figure,as determined by the degree of polarization of an object;

FIG. 3 is a calculated plot of the degree of polarization as a functionof aspect angle in degrees, illustrating the emission polarization formetals and dielectrics;

FIG. 4 is a plot of electrical conductivity as a function of a frequencyin several different media;

FIG. 5 is a plot of degree of emission polarization measured as afunction of aspect angle for polished aluminum and teflon;

FIG. 6 is an alternate embodiment of a systme for measuring the degreeof polarization of an object using a circular polarizer;

FIG. 7 is an optics diagram of a laser scanning system for objectidentification by means of the degree of scattering of polarized lightilluminating the object; and

FIG. 8 is a block diagram of a video processor system for the objectidentification technique illustrated in FIG. 7.

Referring to FIG. 1, an object target of unknown compositioncontinuously emits radiation which is incident upon infrared detectors12, 14 and 16 after passing through polarized filters 18, 20 and 22,respectively. The object target 10 may be in a field of view for thedetectors that includes both metallic and nonmetallic objects. Aselector wheel 24 rotated by means of an electric motor 26 passes orblocks the radiation emitted from the object target 10 to the detectors12, 14 and 16. In the position shown, the selector wheel 24 passesradiation emitting from the object target 10 to the detectors. When theselector wheel 24 has been rotated thirty degrees, radiation from thetarget is blocked and radiation from a reference black-body source 28reflects from the background reflector 24a through the polarized filters18, 20 and 22 to the detectors 12, 14 and 16. Thus, the radiationincident upon the detectors 12, 14 and 16 alternates every thirtydegrees of rotation of the wheel 24 between the target 10 and thereference source 28. Many other methods can be used to alternate theradiation incident on the detectors between the target 10 and thereference source 28. The selector wheel 24 is intended as only oneexample.

The reference source 28 emits black-body or thermal radiation which isdefined as the electromagnetic radiation present in any region of emptyspace at thermodynamic equilibrium at temperature T. Black-bodyradiation is isotropic and unpolarized and has a continuous distributionof frequencies. It is of practical importance to the present inventionas a means for referencing the emitted radiation from the target 10.

Each of the polarization filters 18, 20 and 22 is oriented at adifferent angle from the others to pass only light polarized along apreferred axis to the respective detector in line therewith. There are anumber of polarized filters available; typical of these are the wiregrid polarizers.

An output signal from the detector 12 alternates proportionally betweenthe radiant flux density from the target 10 and the reference source 28along the particular polarization axis of orientation of the polarizedfilter 18. This produces a two-level alternating signal, which afteramplification and rectification in an amplifier 30, produces a D.C.signal on the line 32 proportional the radiation power differencebetween the target 10 and the reference source 28 independent ofextraneous power sources. The operation of the amplifier 30 issynchronized with the motor 26 to give the correct sign to the D.C.signal on line 32 by means of a connection 34. An adjustable resistor 36provides a means for weighting the power difference signal depending onthe system response and the number of detectors employed in a system todetermine the degree of polarization or radiation emitted from thetarget 10. Similarly, the detector 14 produces an output signalproportional alternately between the radiant flux density from thetarget 10 and the reference source 28 along the polarization axis of thepolarized filter 20. To determine the degree of polarization orradiation from the target 10, the polarized filter 20 is orientated atan angle independent of the polarized filter 18. An amplfier 38 operatesto amplify and rectify the dual level alternating signal resulting fromthe radiation emitted by the reference source 28 and the radiationemitted by the target 10 to produce a radiation differential signal online 40, again independent of extraneous power sources. The operation ofthe amplifier 38 is also synchronized with the motor 26 by means of theinteconnection 34. A variable resistor 42 provides a means for weightingthe output of the amplfier 38 by a factor determined by the systemresponse and the number of detectos used. In the same manner, thedetector 16 produces an output signal proportional alternately betweenthe radiant flux density from the target 10 and the reference source 28along the axis of polarization of a polarized filter 22. The polarizedfilter 22 has the polarization axis oriented at an angle relative to thetarget 10 independent of the angle of orientation of the polarizedfilters 18 and 20. Thus, each of the polarized filters 18, 20 and 22 isoriented along a different axis relative to the target 10. Accordingly,the detectors 12, 14 and 16 view the radiation of the target 10 alongdifferent planes of polarization. An amplifier 44 operates to amplifyand rectify the dual level alternating signal resulting from the emittedradiation of the reference source 28 and the target 10 to produce aradiation differential signal on a line 46 independent of extraneouspower sources. The operation of the amplfier 44 is synchronized with themotor 26 by means of the interconnection 34. A variable resistor 48provides a means for weighting the difference signal on line 46according to the system response and the number of detectors used.

The weighted output of the amplifier 30 along with the weighted outputsof the amplifiers 38 and 44 are applied to individual input terminals ofa summing network 50. Typically, the summing network 50 and othersumming networks of the system may be operational amplfiers that producean output proportional to the sum of the inputs or an outputproportional to the difference of one or more inputs. The summingnetwork 50 produces a signal on line 52 equal to the difference betweenthe total emitted radiation from the target 10 and the total emittedradiation from the reference source 28. This signal is applied to oneinput of a summing network 54 which has a second input equal to thetotal radiation emitted by the reference source 28 as established at thewiper arm of a potentiometer 56. Since reference source 28 produces aknown output I the voltage proportional to that output may be set by thewiper arm of potentiometer 56. By operation of the summing network 54, asignal is produced or a line 58 equal to the total intensity measured bythe detectors 12, 14 and 16 independent of the radiation emitted by areference source 28. This total emitted radiation signal is applied tothe input of a squaring network 60 which produces a signal on line 62equal to the square of the total radiation emitted by the target 10.

In addition to the summing network 50, the weighted outputs of theamplfiers 30, 38 and 44 are applied to inputs of the differentialsumming network 64. The weighted output of the amplifier 38 is appliedto one input of the network 64 through a voltage divider of resistors 66and 68. Resistors 66 and 68 are sized such that the signal applied tothe network 64 is one-half the weighted output of the amplfier 38.Similarly, the weighted output of the amplifier 44 is applied to oneinput of the network 64 through a voltage divider of resistors 70 and72. This voltage divider also divides the weighted output of theamplfier 44 by one-half. Network 64 differentially combines the outputsof the amplifiers 38 and 44 with the output of the amplifier 30. Anoutput of the network 64 on line 74 is equal to the degree ofpolarization of radiation from the target 10 times the total radiationintensity and a term proportional to the angle of the orientation of thepolarized radition, all divided by two. This signal is applied to theinput of a squaring circuit 76 which has an output on a line 78 appliedto one input ofa summing network 80.

A second input to the network 80 is the output of a squaring circuit 82having an input terminal connected to a divider network includingresistors 84 and 86. The signal applied to the resistors 84 and 86 isthe output of a summing network 88. Network 88 differentially adds theweighted outputs of the amplfiers 38 and 44. By differentially combiningthese two weighted output signals in the network 88 and applying thissignal to the divider network of resistors 84 and 86, a signal isproduced on line 90 equal to the degree of polarization of radiationfrom the target times the total radiation intensity and a termproportional to the plane of the polarized radiation, all divided bytwo.

Summing network 80 combines the output of the squaring circuit on line78 and the output of the squaring circuit 82 on line 92 and produces anoutput signal independent of the orientation of the polarizationemitting from the target 10. This signal is amplified in an amplifier 94which has an output equal to the square of the degree of polarization orradiation emitted from the target times the square of the totalradiation intensity appearing on line 96.

The signals on lines 62 and 96 are applied to separate inputs of adivider network 98 which processes the signals in a manner to produce anoutput signal related to the square of the degree of polarization ofradiation from the target 10.

In operation, thermal radiation emitting from within the target 10 isunpolarized until it crosses the surface interface. As discussedpreviously, the components of the radiation in the parallel andperpendicular emission planes are transmitted differently in crossingthis interface. This difference is a function of the electricalconstants of the material and the direction of emission with respect tothe normal of the target surface, as predicted as Fresnels equation. Inorder to remotely measure the degree of polarization of receivedinfrared radiation, it is necessary to make at least three radiometricintensity measurements with linearly polarized infrared detectors.

Referring to FIG. 2, the polarization ellipse shown represents the paththat an electric vector of radiation sweeps, propogating normal to thefigure plane, for a material having a degree of polarization given bythe equation:

where I, and I are the intensities of the radiation along the semi-minorand semi-major axis, respectively. Since the detectors l2, l4 and 16 areremote to the target 10, the orientation of the polarization ellipse, x,with respect to the sensing system is unknown. Hence,-in general, thereare three unknowns; the degree of polarization, P, the orientation angleX and the total intensity I l,+l,. Accordingly, to determine the degreeof polarization, P, it is necessary to make at least three radiometricintensity measurments with linearly polarized detectors whose planes ofpolarization are oriented at three angles, 0,, 0, and 6 with respect tothe ellipse of FIG. 2, that is, with respect to the surface of thetarget l0.

A linear polarizer whose plane of polarization is oriented at the angle0; will transmit energy at an intensity given by the expression:

I t [I cos (0, x) I, sin (0, x) T l 2+ 1)/ r 0/ 082 t 01 7,1[1 P cos2(0,)]/2 where I, the total intensity transmitted by the linear polarizer, I+1 and 2" 1)/ The factor 7,, represents the attenuation of the radiationby the material of the linear polarizer independent of the angle 0,.Hence, the total transmission of a linear polarizer as a function of theangle of orientation is given by the equation:

= o/ H P cos r *x)] where P is the degree of polarization of theincident radiation.

In order to obtain a radiometric measurement with an infrared detector,it is necessary to have the calibration source 28 of known intensity I,as a refernce for the unknown intensities from the target 10. Thus, the

voltage output of a linearly polarized and calibrated infrared detectorwill be given by the expression:

W 1) R fl t) 0) where R is the responsivity of the detector. Black-bodyreference source 28 will be unpolarized, i.e., P 0, so

that 'r(6,)I,, (0 /2) I, and hence equation 4 can be rewritten:

To simplify let us define a normalized detector output voltage u(02v(0,)/R'r, or

u(0,) II,,+I Pcos2(0;-x)

= Il +I P(cos 20,cos 2x sin20, sin 2 Equation 6 can be written for Nlinearly polarized detectors in matrix form as:

(0.) 1- cos it. 1513b; "I: 1..

M91) 1 cos 20, sin 20;

II cm 2x 11(1),.) 1 cos 26,, sin 20,. I! sin 2x A matrix equation of theform u Al has a least squares solution given by 1 (AT/4)" A u where thesuperscript T denotes the transpose of the matrix. Equation 7 has theleast squares solution:

I 2 cos 29, 2 sin 26, N i N I l l g 2 N $7: sin 2 (C 29, N 2 sin 201which takes on a very simple form if the planes of polarization of thepolarizers 18, 20, and 22 are oriented such that 6,=(i1)1r/N, I= 1,2,.N. With this choice Of 01,

2 cos 20r= 2 sin 26r= 2 sin 26h cos 29|=0 and 2 cos 9|= 2 sin 2th =N/2 II I l Equation 9 reduces to l IPcosZx 0 2 0 zuwocoswl l IPsm2 0 0 2FEII(01)SIH20( The weightersiias dhiii right arsrsquaiibs 10) can beobtained since u(6,) are measured voltages and the quantities cos20, andsin20, are known parameters. Therefore, the three terms on the left sideof Equation 10 can be obtained by the indicated operations. The degreeof polarization of the radiation, P, can then be determined, i.e., if:

P [(IPcosZ (IPsin2 1 n The denominator in Equation 1 1, I is obtained byadding the known refernce signal I,,to the term I I and then squaring.

- Referring again to FIG. 1, voltage output, u(0,), of

' the detectors 12, 14 and 16 are obtained by alternately viewing thetarget 10 and reference source 28. The voltage output u(0,) is thenapplied to the appropriate variable resistor to obtain one-third thenormalized voltage given by equation 6 above, that is, u(0,)/3.

The dividing factor for any system employing multiple detectors todetermine the degree of polarization is where o, 0, 0, and 0, 120.

The known calibration signal, 1,, is added to equation (12a) in thesumming network 54 to obtain the total intensity measurement I which isthen squared in the network 60 to yield 1. Equation (12b) is the outputof the summing network 64 with the right side defining the three inputsthereto. Similarly, equation (120) gives the output of the summingnetwork 88 and defines the input to the squaring network 82. Squaringthe terms on the left of equations (12b) and (12c) and summing 2:40,)cos 2th 2140, sin 20,

them in the network yields the expression Fi /4 which is then amplifiedby a factor of four to give l P This signal is then divided by thesquare of the total intensity, I, in the dividing network 98 to give thesquare of the degree of polarization, P.

Referring to FIG. 3, there is shown a plot of degrees of polarization asa function of aspect angle for a dielectric and metals in the 8 to 14micron wavelength region. The aspect angle is the observation anglerelative to the normal to the surface of the target 10. The metals aredefined by the electrical constants:

o'lwe 10 to I0 and als l where 0' the electrical conductivity,

0) the angular frequency of the emitted radiation from the target 10,and

e and e, are the dielectric constants of the material and free spacerespectively. These electrical constants are considered to berepresentative of metals in the infrared region. The electricalconstants for the dielectric are which are considered representative ofgood dielectrics in the 8 to 14 micron wavelength region.

As shown by the curves of FIG. 3, the degree of polarization ofradiation emitted from an object increases with the aspect angle.However, even at the lower values for the aspect angle, there isconsiderable difference between the degree of polarization of radiationemitted from metal as compared to the dielectric.

FIG. 4 is a plot of the ratio o-lme as a function of frequency forseveral common media on a log-log plot. In the infrared region of 8 to14 microns, this ratio for copper can be seen to be between 10' and 10For dielectrics, however, in the same region, this ratio is minimal.Since the term in the ratio 01m: that most affects its value is thematerial conductivity, it can be shown that the conductivity of thetarget 10 can be related to the degree of polarization of emittedradiation.

Referring to FIG. 5, there is shown a plot of emission polarization at10.17 microns for aluminum and teflon as a function of aspect angle. Thepolished aluminum may be used to direct polarized radiation to adetector 19. An output from the detector 19 is applied to an averagingnetwork 21 and a squaring network 23. After averaging the intensitysignal in the network 21, it is squared in a squaring network 25. Theoutput of the squaring circuit 23, in turn, is averaged in an averagingnetwork 27. Outputs from the averaging network 27 and the squaringnetwork 25 are applied to inputs of a summing network 29. An output ofthe summing network 29 passes through a variable resistor 33 to anoutput terminal 35. In addition to the summing network 29, the output ofthe network 25 appears at the output terminal 31.

If a circular polarizer is used, the following equation for theradiation intensity can be written:

[(2) A I, [l Pcos2(0+wt)] where a) is the angular speed of thepolarizer. If 1(t) is integrated over time, cos2(0+mt) goes to zero. If1(1) is squared,

may be obtained by the operation of the system of FIG. 6 as given by theequation:

In addition to identifying metallic objects from nonmetallic objects bythe degree of polarization of emitted radiation, a smooth object isidentifiable from a rough object by the degree of polarization ofreflected radiation. Referring to FIG. 7, there is shown an opticsdiagram and processing circuitry of an active object identificationsystem. A linear polarized monochromatic coherent light beam 104 from alaser 106 propagates to a first mirror 108 of a mirror pair thatincludes a second mirror 110. These mirrors oscillate in a manner suchthat the light beam 104 is reflected from the mirror 110 along eitherpath 112 or 114. A light beam along the path 112 is incident on a mirror116 of a mirror pair that includes a second mirror 118. A light beamimpinging on the mirror 116 is reflected to the mirror 118 and reflectedtherefrom to a mirror 120. A light beam along the path 114 impinges onthe mirror 118 and is reflected therefrom to the mirror 116 and againreflected to a mirror 122. Reflection patterns on the mirrors 120 and122 are directed to a vertical scanner 124 and then projected to a fieldof view 126 that contains an object target to be identified.

The mirror pair comprising the mirrors 108 and 110 .and the mirror paircomprising the mirrors 1 16 and 118 are part of a magnetostrictivetorsional light beam scanner. Each consists of a torsionally resonant,magnetostrictive tube which is driven at its resonant frequency by asingle turn winding. One end of the tube is clamped in a fixed positionand the other free end is formed into the mirror. The basic scanner iscapable of producing one dimensional sinusiodal deflection functions;however, combinations as illustrated can produce triangular, sawtooth orvarious types of two dimensional scanning functions. A more completeunderstanding of the operation of the mirror pairs along with thevertical scanner 124 to produce a raster scan pattern on the field ofview 126 may be had by referring to U. S. Pat. No. 3,631,254 ofGerald R.Fournier et al., filed Dec. 31, 1968 Ser. No. 788,259, and assigned tothe assignee of the present invention.

Laser radiation reflected from the field of view 126 passes through acollecting lens 128 having a narrow band pass optical coating centeredat the frequency of the light beam generated by the laser 106. From thecollecting lens 128, reflected radiation is directed to a beam splitter130 which passes one portion of the reflected light to a linearlypolarized detector 132 and directs another beam of the reflected energyto a linearly polarized detector 134. The amount of light impinging oneach of the detectors 132 and 134 is determined by the degree ofpolarization of the light reflected from the field of view 126.

Output voltages from the detectors 132 and 134 may be passed through anamplifier in a manner as illustrated in FIG. 1. The output of each ofthese amplifiers (not shown) connects to one input of a video processor136. The video processor 136 considers the two outputs from thedetectors 132 and 134 and generates a display signal to a cathode raytube display 138.

Referring to FIG. 8, there is shown a block diagram of the videoprocessor 136 including input lines 140 and 142 connected to the outputof amplifiers for the detectors 132 and 134, respectively. Voltages onthe lines 140 and 142 are applied to inputs of a summing network 144 andinputs to a differential network 146. An output of the differentialnetwork 146 is applied to a threshold gate circuit 148 which has anoutput terminal connected to an amplifier 150. The output of theamplifier 150 is applied to one input terminal of the summing network152 which has a second input terminal connected to the output terminalof the summing network 144. The summing network 152 produces displaysignals applied to the input of the CRT display 138.

In operation, assume that an object target is illuminated by thelinearly polarized beam from the laser 106 as it scans the field of view126. If the polarization state of this beam is represented by thevectorj, and the reflected radiation, which is sensed by a linearlypolarized detector, such as detectors 132 and 134, whose polarization isrepresented by the vector F, then the voltage output of the detectorwill be given by the expression:

where V,,,, equals the reflected radiation voltage from one of thedetectors 132 or 134. Since the laser 106 produces a linearly polarizedbeam, a coordinate system can be selected such that:

vector of the transmitted beam and the polarization vector of the linearpolarizer. Using this coordinate system and the equation 16, the voltageoutput of a linearly polarized detector can be writted as:

CEFZ W213i) (3) S cost]; S sin p A linear detector output isproportional to the power incident thereon and is hence, proportional tothe time average of V 2 or:

cos( is zero. For a smooth reflector, the scattering matrix reduceswhere S and S are given by Fresnels equation. In this case, the equation(19) reduces to:

For a rough surface target in the field of view 126, there will bemultiple reflections such that the reflected radiation will have randompolarization causing the scattering elements to have the same magnitudeand random phase. In this case, equation (19) reduces to:

which is independent of the angle between the polarization of the laser106 and one of the linearly polarized detectors 132 or 134.

Thus, for a smooth surfaced object, the detectors 132 and 134 will havean output dependent upon their angle of orientation with respect to thepolarized light output of the laser 106. On the other hand, for a roughsurfaced object in the field of view 126, each'of the detectors 132 and134 will produce a constant level output independent of their angle oforientation.

By combining the outputs of the detectors 132 and 134 in a system asillustrated in FIG. 8, the CRT display 138 will display a view of thearea scanned with smooth surface objects enhanced.

While only preferred embodiments of the invention, together withmodifications thereof, have been described in detail herein and shown inthe accompanying drawings, it will be evident that various furthermodifications are possible without departing from the scope of theinvention.

What is claimed is:

l. The method of identifying an object by determining the degree ofpolarization of energy reflected therefrom, comprising the steps of:

illuminating the object with radiant energy polarized along a knownaxis, measuring the radiation reflected from said object by polarizeddetectors orientated at different angles with respect to the knownpolarized axis,

summing the reflected energy measurements as resulting from each of saiddetectors to obtain a total intensity measurement,

generating a signal representing the difference between the totalintensity measurements in excess of a threshold value, and

summing the total intensity measurement and the signal representing thedifference between the intensity measurements above the threshold toproduce a display signal that identifies an object by the degree ofpolarization of the energy reflected therefrom.

2. The method of identifying an object by determining the degree ofpolarization of energy reflected therefrom as set forth in claim 1including the step of scanning a field of view with the polarixed energyilluminating the object to generate background display signals used inthe identification of an object.

3. The method of identifying an object by determining the degree ofpolarization of energy reflected therefrom as set forth in claim 1including the step of splitting the energy reflected from the object tobe identified into a beam for each detector measuring the reflectedenergy.

4. The method of identifying an object by determining the degree ofpolarization of energy reflected therefrom as set forth in claim 1including the step of inputting said display signal that identifies anobject to a cathode ray tube.

5. Apparatus for identifying an object by determining the degre ofpolarization of energy reflected therefrom, comprising:

means for illuminating a target with energy polarized along a knownaxis,

means for scanning a field of view including the object to be identifiedwith the energy polarized along the known axis,

means for measuring the polarized reflected radiation from the scannedarea by at least two detectors orientated at different angles withrespect to the known axis of the polarized energy,

means for summing the reflected energy measurements resulting from eachof the detectors to obtain a total intensity measurement,

means for generating a signal representing the difference between theintensity measurements from each of the several detectors in excess of athreshold, and

means for adding the total intensity measurement signal and the signalin excess of the threshold to produce a display signal for identifyingan object in the scanned field of view.

6. Appraratus for identifying an object by the degree of polarization ofenergy reflected therefrom as set forth in claim 5 wherein saidilluminating means in a source of monochromatic polarized light.

7. Apparatus for identifying an object by the degree of polarization ofenergy reflected therefrom as set forth in claim 6 wherein said sourceis modymium laser.-

8. Apparatus for identifying an object by the degree of polarization ofenergy reflected therefrom as set forth in claim 5 including means forsplitting the reflected radiation into a beam for each detector.

displaying the area viewed.

1. The method of identifying an object by determining the degree ofpolarization of energy reflected therefrom, comprising the steps of:illuminating the object with radiant energy polarized along a knownaxis, measuring the radiation reflected from said object by polarizeddetectors orientated at different angles with respect to the knownpolarized axis, summing the reflected energy measurements as resultingfrom each of said detectors to obtain a total intensity measurement,generating a signal representing the difference between the totalintensity measurements in excess of a threshold value, and summing thetotal intensity measurement and the signal representing the differencebetween the intensity measurements above the threshold to produce adisplay signal that identifies an object by the degree of polarizationof the energy reflected therefrom.
 2. The method of identifying anobject by determining the degree of polarization of energy reflectedtherefrom as set forth in claim 1 including the step of scanning a fieldof view with the polarixed energy illuminating the object to generatebackground display signals used in the identification of an object. 3.The method of identifying an object by determining the degree ofpolarization of energy reflected therefrom as set forth in claim 1including the step of splitting the energy reflected from the object tobe identified into a beam for each detector measuring the reflectedenergy.
 4. The method of identifying an object by determining the degreeof polarization of energy reflected therefrom as set forth in claim 1including the step of inputting said display signal that identifies anobject to a cathode ray tube.
 5. Apparatus for identifying an object bydetermining the degre of polarization of energy reflected therefrom,comprising: means for illuminating a target with energy polarized alonga known axis, means for scanning a field of view including the object tobe identified with the energy polarized along the known axis, means formeasuring the polarized reflected radiation from the scanned area by atleast two detectors orientated at different angles with respect to theknown axis of the polarized energy, means for summing the reflectedenergy measurements resulting from each of the detectors to obtain atotal intensity measurement, means for generating a signal representingthe difference between the intensity measurements from each of theseveral detectors in excess of a threshold, and means for adding thetotal intensity measurement signal and the signal in excess of thethreshold to produce a display signal for identifying an object in thescanned field of view.
 6. Appraratus for identifying an object by thedegree of polarization of energy reflected therefrom as set forth inclaim 5 wherein said illuminating means in a source of monochromaticpolarized light.
 7. Apparatus for identifying an object by the degrEe ofpolarization of energy reflected therefrom as set forth in claim 6wherein said source is modymium laser.
 8. Apparatus for identifying anobject by the degree of polarization of energy reflected therefrom asset forth in claim 5 including means for splitting the reflectedradiation into a beam for each detector.
 9. Apparatus for identifying anobject by the degree of polarization of energy reflected therefrom asset forth in claim 5 wherein said detectors are silicon photo diodes.10. Apparatus for identifying an object by the degree of polarization ofenergy reflected therefrom as set forth in claim 5 including displaymeans responsive to said display signals and including a cathode raytube for displaying the area viewed.